It is particularly important in industrial use of machine tools that they run as free of disturbance as possible with low monitoring and maintenance demands. Any defects in the machine and/or disturbances during machining lead to undesired downtimes and even to demanding and cost-intensive repairs connected with economic loss from shutdown of the machine tool. A disturbance of a special type in this context is undesirable collision between the moving machine parts of the machine tool, for example, the tool or work table and the work piece being machined, as well as machine parts situated in their surroundings, like clamping devices to attach the work piece to the work table, parts of the work table itself or of the machine frame, devices to supply current to the work zone, rinsing agents, devices for smoke and dust removal, protruding parts of the work piece being machined or work pieces already machined on a pallet, especially drop-out pieces. The mentioned clamping devices are particularly critical and restrictive for motion freedom of the tool. Depending on the size and number of work pieces clamped in the work zone, differently shaped clamping devices in different numbers are arranged in the work zone and all are not fully examinable by the operating personal, for which reason collisions can occur from incorrect operation manual operation or incorrect programming in automatic operation.
Various methods to protect against such interferences, especially collision protection methods, are known from the prior art. However, they do not offer a satisfactory solution in terms of practical suitability, cost/benefit ratio and long-term behavior.
Preventive methods wherein the machine tool is equipped with collision detectors, which are connected to the machine control, to recognize any hazardous situation, and, when necessary, stop the advance movement of the tool electrode are among known collision protection methods. An example of such a method is described in U.S. Pat. No. 5,118,914, wherein a pressure sensor arranged on the tool head of the machine in the event of a collision initiates interruption of relative movement between the machine head and the obstacle when a stipulated threshold pressure is surpassed. Comparable collision recognition by means of a capacitive sensor arranged on the work head is known from Japanese Publication JP 6-4206. Scanning systems in the work space of a machine tool also belong in the category of preventive collision protection systems, as well as image-processing systems, and are based on the principle of radiation reflection. These preventive methods, however, have the drawback that they are relatively expensive because of the required additional sensors and are unsuitable with reference to long-term behavior because of soiling or wear of the sensors.
On the other hand, reactive disturbance protection methods are also known. In these methods a disturbance (e.g., a collision situation) occurs first and interruption of advance movement is then immediately initiated via the machine control. In a known method of this category in the field of electric discharge machining (EDM), the drive current fed to the machining electrode is monitored during machining and on occurrence of a sharp rise in drive current, which indicates a collision, interruption of axial movements is initiated.
Another collision protection method is taught in German reference DE 196 00 538 A1 in conjunction with EDM in which the machine operator establishes forbidden and permitted zones in the control before machining so that the control only permits advance movements into the permitted zones in which it is assumed that no collisions occur. This method is relatively cost-effective, but requires from the machine operator a high degree of readiness and attention in order to correctly program the permitted zones. However, in complex machining, especially of several work pieces, this is not always possible with absolute correctness.
The combination of direct and indirect position measurement systems is known from German reference DE 34 26 863 A1 in conjunction with position regulation of a positioning drive of a numerically controlled machine tool. A first direct length measurement system is then arranged on the machine part being moved by a drive motor and an indirect position measurement system mechanically coupled to the drive motor is additionally provided. The measured values of the two position measurement systems are used for different improvements in the context of position regulation, such as accurate stop by backup of the indirect measurement with higher resolution and an increase in control accuracy in the context of interpolation, for example. However, there is no mention of applications in the direction of disturbance sensing.